1. Field of the Invention
This invention relates generally to a method of producing a flip chip package and resulting structure, and more specifically, to a method that produces a low stress flip chip package that is resistant to thermal fatigue and the resulting structure.
2. Description of Related Art
One type of flip chip package includes one or more chips mounted on a substrate, one type of which is conventionally termed a laminate chip carrier. The laminate chip carrier is usually mounted on a printed circuit card or circuit board. In the xe2x80x9cflip-chipxe2x80x9d configuration, the chip is mounted with electrical contacts thereon, mounted onto solderable metal pads of the laminate chip carrier, using solder balls, sometimes referred to as solder joints.
A flip-chip package typically requires an underfill material to reinforce the solder joints that are prone to fatigue failure due to thermal cycling. The underfill material encompasses the solder interconnections between the chip and the laminate chip carrier.
In an effort to increase thermal dissipation and flatness of the flip-chip package, the packages frequently include a conductive stiffener mounted on the laminate chip carrier and a cover plate mounted on the chip. The stiffener, usually comprised of metal, is preferably mounted onto the laminate chip carrier such that the stiffener surrounds the perimeter of the chip and is preferably about the same height as the chip. The cover plate typically has a surface area greater than the chip, and the cover plate is generally centered on the chip with the overhang of the cover plate resting on the stiffener. In conventional processes, the cover plate is added after the soldered connections have been made, and after the underfill material has been applied and cured. The cover plate is mounted onto the chip and stiffener typically with the application of an adhesives. The adhesive, in addition to adhering the cover plate to the chip and stiffener, is typically a conductive adhesive that improves the transfer of heat from the chip to the cover plate.
A well-known problem with the flip-chip packages, in general, is thermal fatigue failure. The differences in the coefficient of thermal expansion (CTE) of the flip-chip package materials, that is, the chip, laminate substrate, stiffener, cover plate, underfill and adhesive, causes is increased stress within the package. During the operation of the flip-chip package, thermally induced stress causes the package to bend and warp, and may cause the underfill or adhesive to delaminate, the soldered joints to fracture, or the chip to crack. Soldered connections that connect the flip-chip package to the circuit card or circuit board are also susceptible to thermal fatigue.
It is desirable to produce a flip-chip package that has reduced stress throughout the temperature range encountered in manufacture and in use to enhance fatigue life It is also desirable to produce a flip chip package that can withstand wider temperature ranges. It is also desirable, to produce flip-chip package that exhibits minimal warpage.
The invention herein provides for a flip-chip package and a method of producing a flip-chip package that has enhanced product life through stress balancing. The method of producing a flip-chip package of the invention herein comprises the steps of: providing a laminate dielectric chip carrier having electrical circuitry disposed within and preferably a stiffener mounted onto a portion of the laminate chip carrier mounting a chip onto the laminate chip carrier and electrically connecting the electrical contacts on a first or active surface of the chip to the circuitry of the laminate chip carrier; preferably cleaning the first surface of the chip and the portion of the laminate chip carrier directly opposite the first surface of the chip; applying an underfill material between the chip and the laminate chip carrier preferably gelling the underfill material from running along the laminate substrate; providing a cover plate and applying an adhesive to the cover plate adjacent to a second surface of chip such that the adhesive contacts the cover plate and the chip to form an assembly; simultaneously curing the underfill material and adhesive until the underfill material and the adhesive are fully cured.
The underfill material is preferably applied as a liquid material, and when cured, it become sufficiently rigid to withstand forces applied by thermal stress. As soon as the underfill material is applied, the underfill material can be gelled by applying heat. The underfill material is gelled when it forms an outer xe2x80x9cskinxe2x80x9d and thus prevents the underfill material from running along the surface of the laminate chip carrier. It is important, however, that the underfill material and the adhesive are cured to a final cure when exposed to the same temperature and pressure profile. The result is a flip-chip package having reduced stress among the flip-chip package components. The internal stresses between the laminate chip carrier, chip, cover plate, adhesive, and underfill are balanced, that is, brought to a near zero stress state, by fully curing the underfill material and the adhesive together under the same temperature and pressure profile. When the adhesive and underfill materials are substantially cured the result is a flip-chip assembly having reduced internal stress and improved flatness. The glass transition temperature (Tg) of the underfill material should be higher than the operating temperature of the flip-chip package to ensure package flatness and long-term reliability.